Means to test rule – I expect no changes (9)
I believe the answer is:
exception
'means to test rule' is the definition.
I can't judge whether this defines the answer.
'i expect no changes' is the wordplay.
'changes' indicates anagramming the letters.
'i'+'expect'+'no'='iexpectno'
'iexpectno' is an anagram of 'EXCEPTION'.
Can you help me to learn more?
(Other definitions for exception that I've seen before include "Something that does not follow the rule" , "Omission" , "It doesn't conform to the general rule" , "Something not following a rule" , "The one that proves the rule" .)